The present invention relates generally to network monitoring systems, and more particularly to methods and apparatus for locating impairments in a hybrid fiber-coax (HFC) network.
Two categories of impairments regularly present themselves within HFC networks. They are (1) common path distortion (CPD), and (2) noise. Sources of noise may be internal to the network or external to the network. The latter is called “ingress”. Transmission difficulties of voice and data traffic over the return path band in an HFC network, due to CPD and/or noise, are well known and have been well documented in the past ten to fifteen years. Over time, in order to meet the increasing demands of more traffic on their networks, operators have moved to higher throughput formats for return transmission from home, e.g., 16 QAM. These higher order modulation signals are inherently more sensitive to CPD as well as noise. Error correction and other schemes have been implemented to minimize this sensitivity, however the problem still exists. Excessive CPD and noise in the return path can cause disruption to services such as voice and data. As such, it is important and of value to the operator of such networks to be able to quickly locate sources of these impairments such that the problem can be repaired and customer disruption can be minimized.
There are several strategies employed by operators in an attempt to manage return path impairments on the network as well as methods used to try to find and fix problems when they occur.
With the goal of managing ingress, some operators choose to install devices such as passive filters (high pass filters, window filters, or step attenuator filters) that reduce ingress coming from the subscriber's home. Another approach is to install Dynamic Ingress Blocking (DIB) systems that attenuate all or portion of the return path signals during periods of idle traffic and allow all signals through when traffic is present. This latter approach effectively blocks ingress when no traffic is present and allows it to enter the network when traffic is present. It must be noted that these approaches are not completely effective and they do not attempt to locate or eliminate sources of noise and ingress. Many operators alternatively choose not to use any noise mitigating devices.
In order to find and fix problems, many operators have installed return path spectrum analysis monitoring systems. When a noise or ingress problem is measured at the headend, a typical process is for a technician to go into the field and troubleshoot directly by temporarily disconnecting a portion of the return plant while simultaneously monitoring the effect this has at the headend. If ingress or other noise decreases at the headend when a return leg is pulled, a conclusion can be made that this was the direction in which the ingress or other noise was coming from. This trial and error process continues until the source of noise or ingress is found, which can be extremely time consuming.
Other employed systems include the Hunter® System by Arcom Digital, LLC, Syracuse, N.Y., described in U.S. Published Application No. US 2006/0248564 A1, published on Nov. 2, 2006, or one described in U.S. Published Application No. US 2004/0245995 A1, published on Dec. 9, 2004. These systems use passive detection techniques and correlation processing to compare simulated CPD signals generated from forward path signals at the headend to signals measured from the return path. These systems can pinpoint sources of CPD in an HFC network by calculating the distance to the source from a time delay determined from the correlation processing. However, due to inherent inaccuracies of network maps utilized in the cable television industry, further efforts to locate the source are often required. These additional efforts can be time consuming.
When an impairment is measured at the headend, the network operator can use a variety of methods in an attempt to find the cause of the problem. In a typical node on an HFC network, there are many branches and hundreds of devices, each of which could be the cause of the problem. Localizing the source of the problem can be a time consuming endeavor. Some cable systems utilize low attenuation value switches (termed “wink” switches) that attenuate ingress and other noise signals in various portions of the plant to assist the user in localizing the source of noise and ingress. Each wink switch has a unique address, and the various switches within the node are sequentially addressed and turned on such that a few dB of additional attenuation is introduced onto the return path of the leg of the network where the switch is attached. The return path spectrum at the headend is monitored while this switching is occurring—thereby pointing to a particular leg if the timing of when a switch is turned on corresponds to a noise level drop by a corresponding amount at the headend. Another technique employs a field spectrum analyzer in an attempt to troubleshoot the source. Other system operators use dipole antennas installed on trucks that are driven around the system in an attempt to triangulate a source. Others may use the previously mentioned Hunter® System.
The methods described above have deficiencies. In either the DIB or wink switch approach, it is necessary to introduce additional carriers transporting data over the return path in order to address the devices. In addition, the addressable devices installed in the field are complicated and relatively expensive. In the case of the DIB approach, it is required that additional devices be installed in the cable network. This is an expensive procedure that requires system downtime and service interruption. These additional devices also become potential new sources of network problems. The power consumption of such devices can make them unusable in some applications, or could necessitate relocation or adding power supplies to the network. In the case of wink switches, they are only installed in certain active devices; thus, the number of such switches may be limited in the network. Finally, in the case of wink switches, the amount of attenuation of the switch (typically 3 or 6 dB) could be disruptive to network traffic.
In this specification, the term “encoder,” “encoder device,” “ID encoder” or “probe” refers to a device that is installed in the cable network at an appropriate location, and which imparts or encodes an ID code, e.g., a frequency division or code division ID (as further described below), on the signal traffic in the network, so that the location of the device and of any impairments originating downstream from the device can be determined. The device is also sometimes referred to as a “marker” or “marker device” because it can be installed to mark or flag the network leg or branch from which the impairment originates.